1. Field of the Invention
The invention is directed to an improved injection valve for an internal combustion engine.
2. Description of the Prior Art
A valve of the type with which this invention is concerned is known in the industry and is used particularly in conjunction with common rail injection systems for Diesel internal combustion engines. In such an injection valve, a valve control piston is at least partly surrounded by a chamber that contains fuel and communicates with a high-pressure connection. One end of the valve control piston is embodied in the form of a needle and cooperates with a correspondingly embodied valve seat. Depending on the position of the valve control piston, it is thus possible, via an opening located in the chamber surrounding the valve control chamber and leading to a combustion chamber of the engine, to control the fuel injection into the combustion chamber. The position of the valve control piston is defined via the pressure prevailing in the valve control chamber. The pressure prevailing in the valve control chamber is controlled in turn by means of the valve control unit, which for actuation can be operatively connected for instance to a piezoelectric actuator unit.
In the injection valve of the type defined above, in which the valve control unit itself is embodied in valvelike fashion and has a valve closing member which cooperates with a valve seat, fuel is injected into the combustion chamber when the valve closing member is in the open position, and the pressure prevailing in the valve control chamber is thus reduced, as a result of which the valve control piston uncovers the opening leading to the combustion chamber. Conversely, the valve control piston closes the opening leading to the combustion chamber when the valve closing member rests by positive engagement on the valve seat and the so-called common rail pressure is established in the valve control chamber.
The valve control chamber itself has an inlet throttle, disposed in an inlet conduit, and by way of this throttle fuel can be carried into the valve control chamber. The inlet throttle provides that when the valve closing member opens, a pressure equalization does not occur abruptly in the valve control chamber; instead, the pressure equalization comes about only after the valve closing member has closed. Otherwise, the valve control piston could not be moved by way of the pressure prevailing in the valve control chamber. Moreover, the valve control chamber of the injection valve in the prior art has an outlet throttle, disposed in an outlet conduit, that leads to a valve chamber in which the valve closing member is disposed. This outlet throttle serves to provide that the fuel carried away by way of it and returned to a fuel tank via a return line will not flow at the common rail pressure into the return line, because that would result in excessive leakage losses.
In the known injection valve, the valve closing member is actuated by means of an actuating piston, which cooperates via a hydraulic booster with an adjusting piston actuated by means of the actuator. Particularly in a valve closing member actuated by means of a piezoelectric actuator, the closing speed of the piezoelectric actuator is considerably less than its opening speed, since the hydraulic booster is unable to return the valve closing member. Instead, as a rule this return is effected via the fuel pressure prevailing in the outlet throttle, in conjunction with a restoring spring. However, the restoring speed is relatively slight.